Continuously variable hydro-mechanical transmissions are used in a variety of work machines, including for construction, earth moving, forestry, and agriculture. Reference in this regard, Weeramantry, U.S. Pat. No. 7,063,638 B2, issued Jun. 20, 2006, which discloses a representative continuously variable hydro-mechanical transmission. Typically, a continuously variable hydro-mechanical transmission will have a hydrostatic unit as one power input to a planetary gear set, and a mechanical connection to the engine of the machine as a second power input, with the output of the planetary connected via a clutch to one or more final gear reductions in connection with a load, e.g., the wheels, tracks or other drivers of the machine.
An advantage of continuously variable hydro-mechanical transmissions is that they can provide a large speed range seamlessly. As another advantage, continuously variable hydro-mechanical transmissions are typically capable of lower gear ratios than transmissions with fixed gear ratios. As result, the engine and transmission combination can produce higher torques to the wheels, tracks, or other drivers, which is beneficial as it enables the work machine to pull harder. However, the higher torque can damage mechanical aspects of the transmission, particularly, the final gear reduction or output member of the driveline of the transmission. Typically, it is been found that damage to the final gear reduction or output member will occur if the torque is too high for a prolonged period of time.
In agricultural applications, such as wherein a work vehicle such as when a tractor is pulling a large implement, or a deep subsurface tillage tool, a heavy wagon or cart, or the like, potentially damaging continuous high torque loads can be placed on the transmission driveline. Damage from intermittent or incidental high loads can also result from ground conditions, e.g., inclines, ruts, deep furrows, wet spots, transitions onto roads, and the like, when driving, and from contact with denser soil, buried objects such as stones or rock formations, large roots, and the like when doing subsurface tillage.
To avoid such damage, one alternative is to limit engine torque output. However, often the engine supplies power to other systems of the work machine, e.g., auxiliary hydraulics, power take offs, and the like, and it can be problematic to reduce toque output to those systems also. The torque loads of these other systems typically vary and may be unknown, making accurately adjusting engine torque difficult. As another alternative, the transmission torque can be determined using an estimate of the engine torque and subtracting the torque loads of the other systems, or using maximum torque values for those systems, but this is often more complex, more costly and less accurate than desired.
Thus, what is sought is a manner of determining driveline torque of a continuously variable hydro-mechanical transmission of a work machine, particularly in the vulnerable final gear reduction of the driveline, and limiting the torque for preventing damage to the transmission, without the shortcomings set forth above.